A gas turbine engine includes a compressor having one or more stages of rotating blades for compressing air entering the engine. The compressed air enters an annular combustor where a fuel and air mixture is ignited. Hot gases leaving the combustor provide propulsive force for the engine and power a turbine, also having one or more stages of rotating blades. The turbine stages are connected to corresponding compressor stages by respective interconnecting shafts such that the turbine powers the compressor.
The gas turbine engine requires the air exiting the compressor to be distributed to annular channels located radially inwardly and outwardly of the combustor. Conventionally, a diffuser is used to effect such distribution.
The compressed air discharged from the compressor flows at a relatively high velocity and conventionally a pre-diffuser is utilised for initially decreasing the velocity of the compressed airflow to minimise subsequent pressure losses. The pre-diffuser is generally annular, including radially outer and radially inner walls between which the air flows. The radially outer wall is generally frustoconical, flaring outwardly in the downstream direction towards the combustor. The radially inner wall is also generally frustoconical but narrows in the downstream direction. The radially outer and radially inner walls thus diverge away from one another in the downstream direction, such that the area of an inlet of the pre-diffuser is smaller than the area of its outlet. The ratio between the pre-diffuser inlet and the pre-diffuser outlet is typically around 1.5. As the air enters the pre-diffuser, its flow velocity therefore reduces, the larger the area ratio of the outlet to the inlet, the lower the velocity of the air leaving the pre-diffuser. The air leaving the pre-diffuser enters a “dump region” where further deceleration occurs before the air is directed to the annular channels surrounding the combustor.
In a conventional gas turbine engine, around 40% of the air leaving the compressor is passed to the radially outer annular channel (annulus) around the combustor. A further 40% is passed to the radially inner annulus of the combustor. Some of this air is subsequently passed through mixing ports in the combustor walls to thereby enter the combustor for mixing with fuel and burning, some of the air is used for cooling the combustor walls and for passing to the downstream turbines, also for cooling purposes. The remaining 20% of the airflow is passed directly into the combustor at an upstream end thereof.
The air flows that feed the combustor annuli originate from the root and tip regions of the compressor, and flow through the radially outer and inner parts of the pre-diffuser. This air tends to suffer pressure losses along the walls of the pre-diffuser, with most losses occurring in a boundary layer adjacent to those walls. The boundary layer is relatively thin and, where 40% of the airflow is passed to each annuli, the effect of this pressure loss is not very significant because overall pressure losses in the pre-diffuser are low.
However it is now proposed that, to deliver engines that produce reduced NOx emissions, lean burn combustion processes should be used. These processes involve passing much less air down the annuli. In a lean burn combustor, the annuli typically only take around 15% of the compressor delivery air per annulus. There is a danger that much of these small amounts of annulus airflow will come from the root and tip regions of the high pressure compressor. This is the air which suffers pressure losses in the pre-diffuser, being air from the boundary layers. This may result in there being an apparent high pre-diffuser loss from the compressor exit to the combustor annuli, when compared to conventional rich burn combustors with much larger annulus flows.